Many molded plastic articles are formed so as to include a cosmetic outer (or forward) surface and a noncosmetic inner (or rear/rearward) surface. Such a configuration allows a molded plastic article to be mounted to another structure (e.g., an underlying support structure, such as a frame) by means of the non-cosmetic rear surface without adversely affecting (e.g., damaging) the cosmetic forward surface. If in the form of a molded plastic housing (e.g., a computer or mobile phone housing), separate items, such as electronic components, may be secured to the inner non-cosmetic surface, without adversely affecting the outer cosmetic surface of the housing.
To assist mounting a molded plastic article to an underlying structure, or to secure items to the inside of a molded plastic housing, the rear or inner surface of the molded plastic article typically has molded extensions, e.g., molded mounting extensions such as posts and bosses, extending therefrom. For example, the molded article may be mounted on an underlying frame by means of fasteners, such as screws, passing through the frame and into the molded mounting extensions.
In many applications and industries in which molded plastic articles are used, it is desirable to reduce weight without compromising the structural integrity of the molded plastic article. Such weight sensitive applications and industries include, for example, transportation industries, such as the ground, marine, rail and air transportation industries, electronic industries, such as the computer, mobile phone and personal digital assistant industries, and construction industries (e.g., interior and exterior wall panels, and structure fascias). The weight of a molded plastic article may be reduced by reducing the amount of plastic material used to fabricate the molded article. Typically, the weight of a molded plastic article is reduced by making it thinner. To maintain the same level of dimensional stability and structural integrity relative to a thicker molded article, thinner molded articles are typically designed to include structural supports (e.g., ribs) extending from the non-cosmetic rear surface thereof.
Extensions, such as structural and/or mounting supports, extending from the rear surface of a molded plastic article are often undesirably accompanied by defects, such as sink marks in the outer or cosmetic forward surface of the molded article, or warping defects throughout at least a portion of the molded article. Sink mark defects are typically located on the forward surface opposite of the rearward extension. The defects are generally undesirable because of an accompanying reduction in cosmetic appearance (e.g., in the case of sink marks), and structural integrity (e.g., in the case of warping defects).
It would be desirable to develop new plastic molding methods that result in the formation of molded plastic articles having molded extensions, such as structural supports, extending from one surface, that are free of defects, such as sink marks, in the opposite surface thereof. It would be further desirable to develop molded plastic articles having molded extensions extending from one surface, and which are free of defects (e.g., sink marks) in the opposite surface thereof.
U.S. Pat. No. 6,551,540 B1 discloses a method of forming a molded vehicle component having structural ribs, that is free of visible sink marks. The method of US '540 is disclosed as involving first forming a primary vehicle component having structural ribs by injecting thermoplastic material into a first mold cavity. The mold is moved to form a second mold cavity, and additional thermoplastic material is injected therein so as to overmold the front face of the primary vehicle component, thus covering any sink marks in the front face.
U.S. Pat. No. 4,339,408 discloses a method of molding a hollow, plastic article having ribs extending from its walls. The method of US '408 involves first injecting plastic material into a mold to form a wall portion, and allowing the wall portion to cool in the mold. Next, additional plastic is injected into the rib forming cavity of the mold, the plastic material of the ribs is allowed to completely set, and the molded article having ribs is ejected from the mold.
U.S. Pat. No. 5,089,206 discloses a dual charge thermosetting compression molding method by which a molded article having ribs, and no sink mark defects, is formed. The method of US '206 is disclosed as involving placing a first charge of thermosetting plastic material that constitutes ribs into a mold, compressing the first charge, and opening the mold prior to complete cure of the first charge. A second charge of thermosetting plastic material is then placed in the mold, compressed, and the molded article is ejected from the mold. US '206 teaches that after the first compression, it is critical that residual unreacted monomers and oligomers remain in the first charge so as to chemically react with the second charge of plastic material, thereby forming a strong bond there-between. United States Patent Application Publication No. US 2002/0172803 A1 discloses a method of forming a molded article having at least one compression molded plastic layer and at least one plastic projection extending from and bonded to the inner surface of the compression molded plastic layer. The method of US Application '803 involves first forming a plastic layer by compression molding, allowing the compression molded plastic layer to cool, and then forming plastic projections on the back surface of the compression molded plastic layer by injection molding methods.